Going Viral
David had positioned a little cement camouflaged by a flowerbed right mostly on the side in which the neighbors would have turned or (some) cut through the property. After David found the right method, David knew the plan has worked. They can now no longer devastate his estate.
Other Ideas
A few of his insensitive neighbors dented their vehicles without looking twice while they cut through the edge. They now have to learn it the hard way, and therefore they finally learned to stay far away from his estate. The enraged knock on his door was over. The fight was not over; however, when one of them made the decision to hire an attorney to figure out how to stop David.